1. Field of Invention
There is a widespread need for security papers which cannot be counterfeited by photocopy methods. Banking institutions, security firms, and government agencies are alarmed by the capability of present-day copiers such as the Xerox 6500 Color Copier to make faithful color reproductions of bank drafts, stocks and bonds, currency, and the like. As a specific example, dollar bills have been copied and the copies successfully used in bill changers. Commercial printers of checks are actively searching for methods of producing acceptably attractive checks which cannot be faithfully reproduced by color copiers and any reproduction is readily discernible as a counterfeit.
2. Prior Art
Several approaches have been used to prevent the reproduction of documents. Godlewski et al., U.S. Pat. No. 3,852,088 uses a special ink for printing on indicia which does not copy well in the Xerox 2400 or the IBM Copier. This depends on the novel combination of the ink and background color of the document. Braun, U.S. Pat. No. 3,831,007 describes documents which are made non-reproducible through the use of groups of parallel lines combined with other groups of parallel lines, which groups are not parallel to each other. The spacings between the lines are such that the sensors in the photocopiers will not operate to give faithful reproduction of the lined original. Ludlow Paper Co., Needham, Mass., has manufactured a paper called Copy-Trol which is claimed to be copy-proof. The paper is deep cocoa in color. Fitchburg Paper Co., Fitchburg, Mass., has also manufactured a similar type paper which they sell under the name of "No Copy" paper. These processes result in products which are not readily accepted by the banking and securities industries. The esthetics of the products are not attractive due to color, distracting lines and other markings, lack of clarity in printing etc.
Since the lens of the Xerox 6500 will copy and reproduce a screen of 100 lines per inch, but will not produce a screen of 200 lines per inch, Xerox recommends using two different screen frequencies in the background of a document. The word "Void" is screened at 100 lines and the balance of the background at 200 lines. When the document is copied, the 200 line screen will drop out and leave only the word "Void" in the background of the copied document. Two problems result from this method. The word "Void" is noticeable on the document due to the different frequency screens. This is objectionable to the banks and may also result in the original being considered fraudulent. The second problem is the difficulty of producing and holding a good quality 200 line screen on rotary wet offset or litho presses.
It would be desirable to produce documents, which are esthetically attractive, without lines or other matter distracting from the message on the document. It is further desirable to not have the invalidating code appear on the original document. A further desired property is the direct appearance of an invalidating code such as "Void" clearly discernible on a fraudulent copy.